Introduction
Because of the ubiquitous availability of sodium resources and its relatively low electrode potential, rechargeable sodium batteries have been extensively studied for energy storage applications. However, a major obstacle to realize sodium batteries is the absence of suitable negative electrode materials. Titanium-based layered materials, e.g., P3-type Na0.58Cr0.58Ti0.42O2,1 are proposed as potential candidates for this purpose. Nevertheless, when poly(vinylidene fluoride), PVdF, is used as a binder, large irreversible capacities for initial cycle are observed, which hinders its use for practical applications. In this study, instead of PVdF, acrylonitrile-grafted poly(vinyl alcohol) copolymer, PVA-g-PAN,2 is used as the binder for P3-type Na0.58Cr0.58Ti0.42O2. PVA-g-PAN is a non-fluorine polymer with a branched structure prepared by graft polymerization, and the branched structure associated with PAN side chains is beneficial for better dispersion of nanosize carbon and higher coatability to active materials.2 For comparison, acrylonitrile and ether-based monomers grafted PVA, PVA-g-P(AN-co-E), is also prepared and is used as the binder. This polymer has better interaction with electrolyte associated with the presence of negatively charged oxygen species. By using these three different polymer binders, factors affecting electrode performance of Ti-based composite electrode are discussed in detail.
Experimental
Ti-based layered oxides were used as negative electrode materials, and the samples were prepared by conventional calcination method. Composite electrodes with carbon and binder were prepared with AB (HS-100, Denka) as the conductive agent and PVdF (#1100, Kureha), PVA-g-PAN, and PVA-g-P(AN-co-E) as the binders. Electrochemical properties of these composite electrodes were evaluated in two-electrode cells (Type TJ-AC, Tomcell, Japan).
Results and discussion
Electrochemical properties of the P3-type Na0.58Cr0.58Ti0.42O2 composite electrodes with three different binders in Na cells are compared in Fig. 1. PVA-g-PAN and PVA-g-P(AN-co-E) are effective binders, and small irreversible capacities on initial charge is observed when compared with the electrode with PVdF. Our study on polymer binders reveals that the large initial irreversible capacity with PVdF originates from defluorination of PVdF on electrochemical reduction which is the unique chemistry in Na cells, and is not observed for Li cells.
Because electrode resistance with PVA-g-PAN is much lower than that with PVdF, a rapid charge test was conducted. The composite electrode with PVA-g-PAN delivers a discharge capacity of >70 mA h g-1 even by constant voltage charge only for 1 min as shown in Fig. 2. Furthermore, >90% of discharge capacity is obtained by constant voltage charge for 3 min. Excellent rapid charge performance originates from that conductive carbon is more uniformly dispersed for PVA-g-PAN associated with the branched structure of polymer and better interaction with carbon. Together with the results of PVA-g-P(AN-co-E), we further discuss factors affecting the electrochemical properties of Ti-based negative electrode materials with different functional binders in detail.
References
1) Y. Tsuchiya et al. and N. Yabuuchi, Chemistry of Materials, 28, 7066 (2016).
2) S. Tanaka et al. and N. Yabuuchi, Journal of Power Sources, 358, 121 (2017).
Figure 1
